Known processes for obtaining coumarin or a derivative thereof include cyclization and dehydrogenation of a 3-(2-cyclohexanoyl)propionic acid ester or a derivative thereof in the presence of a hydrogenation-dehydrogenation catalyst, e.g., palladium, as disclosed in U.S. Pat. No. 3,442,910 or in the presence of a noble metal catalyst, e.g., palladium, in combination with a co-catalyst, such as barium sulfate or nickel oxide, as disclosed in JP-A-60-181082 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
It is also known to obtain coumarin or a derivative thereof by dehydrogenation of 3,4-dihydrocoumarin or a derivative thereof in the presence of a noble metal catalyst, such as palladium, as disclosed in Ber., 70B, pp. 735-738 (1936) or by using chlorine, bromine, oxygen, or sulfur as disclosed in Monatsh., Vol. 34, pp. 1671-1672 (1913) and German Patent 276,667.
The cyclization and dehydrogenation of a 3-(2-cyclohexanoyl)propionic ester in the presence of a palladium catalyst under heating does not always attain a high yield of coumarin and is accompanied with by-production of 3,4-dihydrocoumarin in a large proportion. The dehydrogenation of 3,4-dihydrocoumarin using chlorine, sulfur, etc. meets difficulty in removing the chlorine or sulfur, etc. from the reaction mixture and therefore involves complicated purification steps.
The dehydrogenation of 3,4-dihydrocoumarin in the presence of a palladium catalyst has an advantage of relatively easy purification. However, hydrogen which is by-produced by the dehydrogenation concurrently induces hydrogenation of coumarin produced into 3,4-dihydrocoumarin, the dehydrogenation as purposed does not always proceed sufficiently, and the yield of coumarin is not so high as expected.